1. Field of the Invention
This invention relates to an improved feed forward circuit and to a method for aligning and balancing the feed forward circuit and, more particularly, to an improved feed forward circuit which can improve the accuracy of the alignment and balancing of the circuit to allow for improved gain flatness and improved distortion reduction characteristics in the circuit.
2. Description of the Prior Art
Various feed forward circuits have been known in the art to provide for distortion reduction of a signal which is amplified as it passes through the circuit. Typically, the feed forward circuit has two cancellation loops, the first isolates noise and distortion and the second produces the distortion cancellation phenomena. If the cancellation process were ideal, the output signal would be in exact replica of the input signal which has been appropriately amplified without the noise and distortion created by amplification in the circuit.
A typical feed forward circuit has a first circuit loop or, a main loop, and a second circuit loop, or error loop. Each loop has amplification means, directional coupler means, and delay means interconnected by common balance circuits and common directional couplers to both loops of the circuit. An signal input terminal and generally a test input terminal are provided in the first circuit loop and a signal output terminal and generally a test output terminal are provided in the second circuit loop. The first circuit loop serves to isolate the noise and distortion created by the amplification means in that loop and the second circuit loop provides reduction of noise and distortion.
Ideally, the feed forward circuit would provide a replica of the input signal, appropriately amplified, without any distortion. The feed forward circuit relies on cancellation to provide noise and distortion reduction. In attempting to provide an output signal which is an amplified replica of the input signal, prior feed forward circuits have been limited by the cancellation capability of the main loop and error loop in the circuit through alignment and balancing of these loops. Inaccurate alignment and balancing also promotes a degredation of gain flatness in the circuit.
For broadband multi-channel operation, two push-pull cascode hybrid integrated radio frequency (RF) amplifiers are typically used in the feed forward circuit, one amplifier in the main loop and one in the error loop of the feed forward circuit. One known method of aligning and balancing a feed forward circuit having radio frequency amplifiers is the termination method. The termination method of alignment and balancing has two basic steps. The first step is to align the balance circuit and adjust the delay means, such as a delay line, in the error loop to provide cancellation in the error loop. This is accomplished by removing the delay means in the main loop, such as a delay line, and replacing this delay line in the main loop with, for example, proper terminating resistors. A signal derived from the test signal generator is then injected at the signal input terminal of the first circuit loop. Initially, the amplification means in the second circuit loop, such as an error amplifier, is turned off to produce a reference signal with no cancellation. Then, the error amplifier is turned on and balancing and delay adjustments are made to produce an output voltage which is a combination of the reference signal and a cancellation signal. The output voltage at the signal output terminal desirably should made as low as possible, thereby indicating a high degree of cancellation.
A disadvantage inherent in this method of aligning and balancing the error loop is that when the delay means, such as a delay line, is placed back into the circuit, a mismatch of impedance occurs between certain directional coupler means and the delay line in the first circuit loop which promotes a change in the transfer characteristics in certain of the directional coupler means. This change in transfer characteristics effects the balance of the previously adjusted error loop of the feed forward circuit. This imbalance limits the ultimate distortion cancellation characteristics of the circuit by limiting the accuracy of the balancing of the error loop. Typically, circuits balanced with this technique will be limited to approximately 22 to 26 dB cancellation, particularly at frequencies above 200 MHz where parasitic and distributed impedances of the circuit elements of the directional coupler means, such as directional couplers, reduce the port-to-port isolation of the directional couplers used in the feed forward circuits.
This typical limitation of 22 to 26 dB cancellation, for example, places constraints on the most important performance characteristic of a feed forward circuit, namely, the reduction of undesired output signals by cancellation in the error loop of the feed forward circuit. If the cancellation in the error loop could be improved by a more accurate alignment and balancing of this portion of the circuit, the distortion reduction capability of the circuit could be improved.
In balancing the main loop of the feed forward circuit by the termination method, a similar process exists as when balancing the error loop by the termination method except that the delay means in the second circuit loop, or error loop, is removed and replaced with, for example, proper terminating resistors. In balancing the main amplifier loop, the main amplifier is first turned off and signal is passed through the signal input terminal of the first circuit loop to provide a reference signal. Then the main amplifier is turned on and balancing and delay adjustments are made to produce an output signal which is a combination of the reference signal and a cancellation signal, the cancellation signal having passed through the amplifier of the main loop, the balance circuitry, and the error amplifier of the second loop to the output signal terminal in the error loop. The output voltage at the signal output terminal of the error loop is a combination of the reference signal and the cancellation signal. The voltage measured for the output signal should be made as low as possible to provide a high degree of cancellation in the main loop. The result of imbalance or imperfect cancellation in the main amplifier loop is degradation of the flatness of the gain versus frequency response of the circuit. Similar impedance matching problems are encountered when the delay means, such as a delay line, is placed back into the circuit which limits the accuracy of balancing of the main loop. Inaccuracy in balancing promotes this degradation of the gain flatness, as described above.
Another known method of aligning and balancing of feed forward circuits is the resistive injection circuit alignment method. In this method, in addition to the signal input terminal in the first circuit loop, or main loop, and the signal output terminal in the second circuit loop, or error loop, an input resistive test terminal is provided in the first circuit loop and an output resistive test terminal is provided in the second circuit loop.
Like the termination method, the resistive injection circuit alignment method also requires two stages of alignment. The first stage of alignment involves balancing and aligning the error loop of the feed forward circuit. In this first stage of alignment a test signal is injected at the input resistive test terminal and the error amplifier in the error loop is turned off to provide a reference signal of a given reference voltage at the signal output terminal of the error loop with the circuit input terminated with the proper resistance. The error amplifier in the error loop is then turned on and the circuit is balanced by adjusting the balance circuitry and certain directional coupler means. With the error amplifier on, in addition to the reference signal, a cancellation signal is also produced at the signal output terminal of the error loop. The magnitude of the cancellation signal is adjusted by adjusting the loss of the balance circuits to attempt to match the magnitude characteristics of the reference voltage. The degree of precision of balancing the circuit is to reduce the magnitude of the combination of the reference signal and the cancellation signal. However, an undesired signal is also present at the signal output terminal in addition to the reference and cancellation signals. The undesired signal proceeds from the input resistive test terminal through the input directional coupler means then through the delay line means of the first circuit loop, the error amplifier of the error loop to the signal output terminal in the error loop. The presence of the undesirable signal at the output terminal presents a serious limitation to the accuracy of the cancellation measurements made by attempting to measure the difference between the reference signal and the cancellation signal to provide a reliable cancellation alignment of, typically 30 dB.
The alignment of the main loop in the feed forward circuit in the resistive injection circuit alignment method is similar to the alignment of the error loop by this method. However, in this method the signal is input to the signal input terminal of the main loop and output is measured at the output resistive test terminal in the error loop with the circuit output terminated with the proper resistance. To obtain a reference signal, the main amplifier in the main loop is turned off and a reference signal which proceeds from the signal input terminal in the main loop through the delay means in the first loop through the error amplifier of the second loop is measured at the output resistive test terminal of the error loop. After the magnitude of this reference signal has been measured, the main amplifier is then turned on and a cancellation signal is also present at the output resistive test terminal. The path of this cancellation signal has proceeded from the signal input terminal in the main loop through the main amplifier in the main loop through, the balance circuitry, and the through the error amplifier in the error loop to provide a cancellation signal at the output resistive test terminal in the error loop. The magnitude of the cancellation signal is adjusted by adjusting the loss of the balancing circuits to attempt to precisely match the magnitude characteristics of the reference signal, such as when balancing the error loop by this method. The degree of precision in balancing the circuit relies upon the capability of the balancing process to reduce the magnitude of the combination of the reference signal and the cancellation signal to a relatively low magnitude as measured at the output resistive test terminal. However, as when balancing the error loop by the resistive injection circuit alignment method, an undesirable signal is also present at the output resistive test terminal. The path of the undesired signal proceeds from the signal input terminal in the main loop through the main amplifier in the main loop through the delay means of the error loop to the output resistive test terminal in the error loop. As in the case in balancing the error loop by this method, a serious limitation is present in regard to the accuracy of the cancellation measurements made by attempting to measure the magnitude of the difference of the reference voltage and the cancellation voltage at the output resitive test terminal to provide a reliable cancellation alignment.
What is needed, therefore, is an improved feed forward circuit and a method for aligning and balancing the same which can provide improved accuracy in the alignment and balancing of the circuit and provide an improved gain flatness and improved distortion reduction characteristics in the feed forward circuit. What is further needed is an improved feed forward circuit and a method for aligning and balancing the circuit which can allow all circuit components to remain in place during alignment and balancing to promote the elimination of impedance matching problems. What is also needed is an improved feed forward circuit and method for aligning and balancing the same which promotes the substantial elimination of undesired signals in the alignment and balancing process to provide for a high degree of accuracy in alignment and balancing of the feed forward circuit.